Apparatus and method for efficient motion estimation

ABSTRACT

The architecture shown can perform global search, local search and local sub pixel search in a parallel or in a pipelined mode. All operations are in a streaming mode without the requirement of external intermediate data storage.

CLAIM OF PRIORITY

This application claims priority under 35 U.S.C 119(e) (1) to Indian Provisional Application No. 20164101546 filed May 4, 2016

TECHNICAL FIELD OF THE INVENTION

The technical field of this invention is image compression.

BACKGROUND OF THE INVENTION

Increasing video resolution and frame rates, along with large number of searching and matching operations involved in motion estimation demand very high performance. While high performance can be achieved by increasing hardware throughput and higher clock frequency, it is important to identify and exploit parallelism present in the algorithm in order to efficiently utilize available hardware resources.

The Motion Estimation process involves searching operations which require accessing large amounts of reference picture data from memory. Memory bandwidth is an expensive resource which often limits the computational parallelism that can be built in hardware. Further, this large data traffic from the memory leads to large power dissipation.

Motion estimation finds a best match for each block in a current video frame among blocks from previously coded frame(s) (called as reference frames). Block size is typically 16×16 pixels.

A widely used metric to define the match is SAD (Sum Of Absolute Difference in all the pixel values of current block and a reference block).

The best match information is indicated by the motion vector: if the current position of a block is (16,16) then motion vector (4,1) means the best match lies at position (20,17) in the reference frame.

The motion vector can also be in fraction pixel precision: half pixel, quarter pixel etc.

Fractional pixels are calculated by interpolating neighboring integer position pixels.

A motion estimation algorithm would typically include these steps:

Stage 1: choosing best among a few predictor motion vectors;

Stage 2: search around winner of Stage 1;

Stage 3, 4: search around winner of Stage 2 and Stage 3 respectively;

Stage 5: sub-pixel search at interpolated positions.

SUMMARY OF THE INVENTION

A parallel motion estimation architecture is shown that enables efficient utilization of computational resources by making use of the inherent parallelism of the motion estimation algorithms.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of this invention are illustrated in the drawings, in which:

FIG. 1 illustrates the organization of a typical digital signal processor to which this invention is applicable (prior art);

FIG. 2 illustrates details of a very long instruction word digital signal processor core suitable for use in Figure 1(prior art);

FIG. 3 illustrates the pipeline stages of the very long instruction word digital signal processor core illustrated in FIG. 2 (prior art);

FIG. 4 illustrates the instruction syntax of the very long instruction word digital signal processor core illustrated in FIG. 2 (prior art);

FIG. 5 illustrates an overview of the video encoding process of the prior art;

FIG. 6 illustrates an overview of the video decoding process of the prior art;

FIG. 7 illustrates an overview of the motion estimation engine of this invention; and

FIG. 8 illustrates one of the local buffers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates the organization of a typical digital signal processor system 100 to which this invention is applicable (prior art). Digital signal processor system 100 includes central processing unit core 110. Central processing unit core 110 includes the data processing portion of digital signal processor system 100. Central processing unit core 110 could be constructed as known in the art and would typically includes a register file, an integer arithmetic logic unit, an integer multiplier and program flow control units. An example of an appropriate central processing unit core is described below in conjunction with FIGS. 2 to 4.

Digital signal processor system 100 includes a number of cache memories. FIG. 1 illustrates a pair of first level caches. Level one instruction cache (L1I) 121 stores instructions used by central processing unit core 110. Central processing unit core 110 first attempts to access any instruction from level one instruction cache 121. Level one data cache (L1D) 123 stores data used by central processing unit core 110. Central processing unit core 110 first attempts to access any required data from level one data cache 123. The two level one caches are backed by a level two unified cache (L2) 130. In the event of a cache miss to level one instruction cache 121 or to level one data cache 123, the requested instruction or data is sought from level two unified cache 130. If the requested instruction or data is stored in level two unified cache 130, then it is supplied to the requesting level one cache for supply to central processing unit core 110. As is known in the art, the requested instruction or data may be simultaneously supplied to both the requesting cache and central processing unit core 110 to speed use.

Level two unified cache 130 is further coupled to higher level memory systems. Digital signal processor system 100 may be a part of a multiprocessor system. The other processors of the multiprocessor system are coupled to level two unified cache 130 via a transfer request bus 141 and a data transfer bus 143. A direct memory access unit 150 provides the connection of digital signal processor system 100 to external memory 161 and external peripherals 169.

FIG. 2 is a block diagram illustrating details of a digital signal processor integrated circuit 200 suitable but not essential for use in this invention (prior art). The digital signal processor integrated circuit 200 includes central processing unit 1, which is a 32-bit eight-way VLIW pipelined processor. Central processing unit 1 is coupled to level one instruction cache 121 included in digital signal processor integrated circuit 200. Digital signal processor integrated circuit 200 also includes level one data cache 123. Digital signal processor integrated circuit 200 also includes peripherals 4 to 9. These peripherals preferably include an external memory interface (EMIF) 4 and a direct memory access (DMA) controller 5. External memory interface (EMIF) 4 preferably supports access to supports synchronous and asynchronous SRAM and synchronous DRAM. Direct memory access (DMA) controller 5 preferably provides 2-channel auto-boot loading direct memory access. These peripherals include power-down logic 6. Power-down logic 6 preferably can halt central processing unit activity, peripheral activity, and phase lock loop (PLL) clock synchronization activity to reduce power consumption. These peripherals also include host ports 7, serial ports 8 and programmable timers 9.

Central processing unit 1 has a 32-bit, byte addressable address space. Internal memory on the same integrated circuit is preferably organized in a data space including level one data cache 123 and a program space including level one instruction cache 121. When off-chip memory is used, preferably these two spaces are unified into a single memory space via the external memory interface (EMIF) 4.

Level one data cache 123 may be internally accessed by central processing unit 1 via two internal ports 3 a and 3 b. Each internal port 3 a and 3 b preferably has 32 bits of data and a 32-bit byte address reach. Level one instruction cache 121 may be internally accessed by central processing unit 1 via a single port 2 a. Port 2 a of level one instruction cache 121 preferably has an instruction-fetch width of 256 bits and a 30-bit word (four bytes) address, equivalent to a 32-bit byte address.

Central processing unit 1 includes program fetch unit 10, instruction dispatch unit 11, instruction decode unit 12 and two data paths 20 and 30. First data path 20 includes four functional units designated L1 unit 22, S1 unit 23, M1 unit 24 and D1 unit 25 and 16 32-bit A registers forming register file 21. Second data path 30 likewise includes four functional units designated L2 unit 32, S2 unit 33, M2 unit 34 and D2 unit 35 and 16 32-bit B registers forming register file 31. The functional units of each data path access the corresponding register file for their operands. There are two cross paths 27 and 37 permitting access to one register in the opposite register file each pipeline stage. Central processing unit 1 includes control registers 13, control logic 14, and test logic 15, emulation logic 16 and interrupt logic 17.

Program fetch unit 10, instruction dispatch unit 11 and instruction decode unit 12 recall instructions from level one instruction cache 121 and deliver up to eight 32-bit instructions to the functional units every instruction cycle. Processing occurs simultaneously in each of the two data paths 20 and 30. As previously described each data path has four corresponding functional units (L, S, M and D) and a corresponding register file containing 16 32-bit registers. Each functional unit is controlled by a 32-bit instruction. The data paths are further described below. A control register file 13 provides the means to configure and control various processor operations.

FIG. 3 illustrates the pipeline stages 300 of digital signal processor core 110 (prior art). These pipeline stages are divided into three groups: fetch group 310; decode group 320; and execute group 330. All instructions in the instruction set flow through the fetch, decode, and execute stages of the pipeline. Fetch group 310 has four phases for all instructions, and decode group 320 has two phases for all instructions. Execute group 330 requires a varying number of phases depending on the type of instruction.

The fetch phases of the fetch group 310 are: Program address generate phase 311 (PG); Program address send phase 312 (PS); Program access ready wait stage 313 (PW); and Program fetch packet receive stage 314 (PR). Digital signal processor core 110 uses a fetch packet (FP) of eight instructions. All eight of the instructions proceed through fetch group 310 together. During PG phase 311, the program address is generated in program fetch unit 10. During PS phase 312, this program address is sent to memory. During PW phase 313, the memory read occurs. Finally during PR phase 314, the fetch packet is received at CPU 1.

The decode phases of decode group 320 are: Instruction dispatch (DP) 321; and Instruction decode (DC) 322. During the DP phase 321, the fetch packets are split into execute packets. Execute packets consist of one or more instructions which are coded to execute in parallel. During DP phase 322, the instructions in an execute packet are assigned to the appropriate functional units. Also during DC phase 322, the source registers, destination registers and associated paths are decoded for the execution of the instructions in the respective functional units.

The execute phases of the execute group 330 are: Execute 1 (E1) 331; Execute 2 (E2) 332; Execute 3 (E3) 333; Execute 4 (E4) 334; and Execute 5 (E5) 335. Different types of instructions require different numbers of these phases to complete. These phases of the pipeline play an important role in understanding the device state at CPU cycle boundaries.

During E1 phase 331, the conditions for the instructions are evaluated and operands are read for all instruction types. For load and store instructions, address generation is performed and address modifications are written to a register file. For branch instructions, branch fetch packet in PG phase 311 is affected. For all single-cycle instructions, the results are written to a register file. All single-cycle instructions complete during the E1 phase 331.

During the E2 phase 332, for load instructions, the address is sent to memory. For store instructions, the address and data are sent to memory. Single-cycle instructions that saturate results set the SAT bit in the control status register (CSR) if saturation occurs. For single cycle 16 by 16 multiply instructions, the results are written to a register file. For M unit non-multiply instructions, the results are written to a register file. All ordinary multiply unit instructions complete during E2 phase 322.

During E3 phase 333, data memory accesses are performed. Any multiply instruction that saturates results sets the SAT bit in the control status register (CSR) if saturation occurs. Store instructions complete during the E3 phase 333.

During E4 phase 334, for load instructions, data is brought to the CPU boundary. For multiply extension instructions, the results are written to a register file. Multiply extension instructions complete during the E4 phase 334.

During E5 phase 335, load instructions write data into a register. Load instructions complete during the E5 phase 335.

FIG. 4 illustrates an example of the instruction coding of instructions used by digital signal processor core 110 (prior art). Each instruction consists of 32 bits and controls the operation of one of the eight functional units. The bit fields are defined as follows. The creg field (bits 29 to 31) is the conditional register field. These bits identify whether the instruction is conditional and identify the predicate register. The z bit (bit 28) indicates whether the predication is based upon zero or not zero in the predicate register. If z=1, the test is for equality with zero. If z=0, the test is for nonzero. The case of creg=0 and z=0 is treated as always true to allow unconditional instruction execution. The creg field is encoded in the instruction opcode as shown in Table 1.

TABLE 1 Conditional creg z Register 31 30 29 28 Unconditional 0 0 0 0 Reserved 0 0 0 1 B0 0 0 1 z B1 0 1 0 z B2 0 1 1 z A1 1 0 0 z A2 1 0 1 z A0 1 1 0 z Reserved 1 1 1 x Note that “z” in the z bit column refers to the zero/not zero comparison selection noted above and “x” is a don't care state. This coding can only specify a subset of the 32 registers in each register file as predicate registers. This selection was made to preserve bits in the instruction coding.

The dst field (bits 23 to 27) specifies one of the 32 registers in the corresponding register file as the destination of the instruction results.

The scr2 field (bits 18 to 22) specifies one of the 32 registers in the corresponding register file as the second source operand.

The scr1/cst field (bits 13 to 17) has several meanings depending on the instruction opcode field (bits 3 to 12). The first meaning specifies one of the 32 registers of the corresponding register file as the first operand. The second meaning is a 5-bit immediate constant. Depending on the instruction type, this is treated as an unsigned integer and zero extended to 32 bits or is treated as a signed integer and sign extended to 32 bits. Lastly, this field can specify one of the 32 registers in the opposite register file if the instruction invokes one of the register file cross paths 27 or 37.

The opcode field (bits 2 to 12) specifies the type of instruction and designates appropriate instruction options. A detailed explanation of this field is beyond the scope of this invention except for the instruction options detailed below.

The s bit (bit 1) designates the data path 20 or 30. If s=0, then data path 20 is selected. This limits the functional unit to L1 unit 22, S1 unit 23, M1 unit 24 and D1 unit 25 and the corresponding register file A 21. Similarly, s=1 selects data path 20 limiting the functional unit to L2 unit 32, S2 unit 33, M2 unit 34 and D2 unit 35 and the corresponding register file B 31.

The p bit (bit 0) marks the execute packets. The p-bit determines whether the instruction executes in parallel with the following instruction. The p-bits are scanned from lower to higher address. If p=1 for the current instruction, then the next instruction executes in parallel with the current instruction. If p=0 for the current instruction, then the next instruction executes in the cycle after the current instruction. All instructions executing in parallel constitute an execute packet. An execute packet can contain up to eight instructions. Each instruction in an execute packet must use a different functional unit.

FIG. 5 illustrates the encoding process 500 of video encoding according to the prior art. Many video encoding standards use similar processes such as represented in FIG. 5. Encoding process 500 begins with the n th (current) frame F_(n) 501. Frequency transform block 502 transforms a macroblock of the pixel data into the spatial frequency domain. This typically involves a discrete cosine transform (DCT). This frequency domain data is quantized in quantization block 503. This quantization typically takes into account the range of data values for the current macroblock. Thus differing macroblocks may have differing quantizations. In accordance with the H.264 standard, in the base profile the macroblock data may be arbitrarily reordered via reorder block 504. As will be explained below, this reordering is reversed upon decoding. Other video encoding standards and the H.264 main profile transmit data for the macroblocks in strict raster scan order. The quantized data is encoded by entropy encoding block 505. Entropy encoding employs fewer bits to encode more frequently used symbols and more bits to encode less frequency used symbols. This process reduces the amount of encoded data that must be transmitted and/or stored. The resulting entropy encoded data is the encoded data stream. This invention concerns content adaptive binary arithmetic coding (CABAC) which will be further described below.

Video encoding standards typically permit two types of predictions. In inter-frame prediction, data is compared with data from the corresponding location of another frame. In intra-frame prediction, data is compared with data from another location in the same frame.

For inter prediction, data from n−1 th (previous) frame F_(n-1) 510 and data from the n th frame F_(n) 501 supply motion estimation block 511. Motion estimation block 511 determines the positions and motion vectors of moving objects within the picture. This motion data is supplied to motion compensation block 512 along with data from n−1 th frame F_(n-1) 510. The resulting motion compensated frame data is selected by switch 513 for application to subtraction unit 506. Subtraction unit 506 subtracts the inter prediction data from switch 513 from the input frame data from n th frame F_(n) 501. Thus frequency transform block 502, quantization block 503, reorder block 504 and entropy encoding block 505 encode the differential data rather than the original frame data. Assuming there is relatively little change from frame to frame, this differential data has a smaller magnitude than the raw frame data. Thus this can be expressed in fewer bits contributing to data compression. This is true even if motion estimation block 511 and motion compensation block 512 find no moving objects to code. If the n th frame F_(n) and the n−1 th frame F_(n-1) are identical, the subtraction unit 506 will produce a string of zeros for data. This data string can be encoded using few bits.

The second type of prediction is intra prediction. Intra prediction predicts a macroblock of the current frame from another macroblock of the current frame. Inverse quantization block 520 receives the quantized data from quantization block 503 and substantially recovers the original frequency domain data. Inverse frequency transform block 521 transforms the frequency domain data from inverse quantization block 520 back to the spatial domain. This spatial domain data supplies one input of addition unit 522, whose function will be further described. Encoding process 500 includes choose intra predication unit 514 to determine whether to implement intra prediction. Choose intra prediction unit 514 receives data from n th frame F_(n) 501 and the output of addition unit 522. Choose intra prediction unit 514 signals intra prediction intra predication unit 515, which also receives the output of addition unit 522. Switch 513 selects the intra prediction output for application to the subtraction input of subtraction units 506 and an addition input of addition unit 522. Intra prediction is based upon the recovered data from inverse quantization block 520 and inverse frequency transform block 521 in order to better match the processing at decoding. If the encoding used the original frame, there might be drift between these processes resulting in growing errors.

Video encoders typically periodically transmit unpredicted frames. In such an event the predicted frame is all 0's. Subtraction unit 506 thus produces data corresponding to the n th frame F_(n) 501 data. Periodic unpredicted or I frames limits any drift between the transmitter coding and the receive decoding. In a video movie a scene change may produce such a large change between adjacent frames that differential coding provides little advantage. Video coding standards typically signal whether a frame is a predicted frame and the type of prediction in the transmitted data stream.

Encoding process 500 includes reconstruction of the frame based upon this recovered data. The output of addition unit 522 supplies deblock filter 523. Deblock filter 523 smoothes artifacts created by the block and macroblock nature of the encoding process. The result is reconstructed frame F′_(n) 524. As shown schematically in FIG. 5, this reconstructed frame F′_(n) 524 becomes the next reference frame F_(n-1) 510.

FIG. 6 illustrates the corresponding decoding process 600. Entropy decode unit 601 receives the encoded data stream. Entropy decode unit 601 recovers the symbols from the entropy encoding of entropy encoding unit 505. This invention is applicable to CABAC decoding. Reorder unit 602 assembles the macroblocks in raster scan order reversing the reordering of reorder unit 504. Inverse quantization block 603 receives the quantized data from reorder unit 602 and substantially recovers the original frequency domain data. Inverse frequency transform block 604 transforms the frequency domain data from inverse quantization block 603 back to the spatial domain. This spatial domain data supplies one input of addition unit 605. The other input of addition input 605 comes from switch 609. In inter prediction mode switch 609 selects the output of motion compensation unit 607. Motion compensation unit 607 receives the reference frame F′_(n-1) 606 and applies the motion compensation computed by motion compensation unit 512 and transmitted in the encoded data stream.

Switch 609 may also select an intra prediction mode. The intra prediction is signaled in the encoded data stream. If this is selected, intra prediction unit 608 forms the predicted data from the output of adder 605 and then applies the intra prediction computed by intra prediction block 515 of the encoding process 500. Addition unit 605 recovers the predicted frame. As previously discussed in conjunction with encoding, it is possible to transmit an unpredicted or I frame. If the data stream signals that a received frame is an I frame, then the predicted frame supplied to addition unit 605 is all 0's.

The output of addition unit 605 supplies the input of deblock filter 610. Deblock filter 610 smoothes artifacts created by the block and macroblock nature of the encoding process. The result is reconstructed frame F′_(n) 611. As shown schematically in FIG. 6, this reconstructed frame F′_(n) 611 becomes the next reference frame F_(n-1) 606.

The deblocking filtering of deblock filter 523 and deblock filter 610 must be the same. This enables the decoding process to accurately reflect the input frame F_(n) 501 without error drift. The H.264 standard has a specific, very detailed decision matrix and corresponding filter operations for this process. The standard deblock filtering is applied to every macroblock in raster scan order. This deblock filtering smooths artifacts created by the block and macroblock nature of the encoding. The filtered macroblock is used as the reference frame in predicted frames in both encoding and decoding. The encoding and decoding apply the identical processing the reconstructed frame to reduce the residual error after prediction.

The architecture shown in this invention enables efficient utilization of computational hardware by making use of the inherent parallelism in the motion estimation algorithm. Two types of parallelism are exploited: (1) For a given current macro-block, search operations in different reference frames are independent of each other and can be performed in parallel (or pipelined fashion). (2) Search operations for two different macro-blocks can be performed in parallel.

Motion estimation algorithms many times involve a global search followed by a local search phase. Global search narrows down the search to one (or more) motion vector candidates among several initial candidates. The local search that comes afterwards involves searching in a region just around the candidate(s) identified during the global search phase. Further, local search typically involves integer motion vector search and fractional (sub-pixel) motion vector search.

The architecture shown in FIG. 7 provides parallel hardware comprising of:

Interface controller 708 connected to processing unit 711, skip engine 706, skip input buffer 707, I/O management unit 710, predictor engine 701, vector SAD engine 702 and subpel engine 703.

Programming interface 709 is connected to skip engine 706 and skip buffer 705.

I/O management unit 710 is connected to interface controller 708, predictor engine 701, vector SAD engine 702 and subpel engine 703.

Processing unit 711 is connected to interface controller 708, predictor engine 701, vector SAD engine 702, subpel engine 703 and interpolation reference buffer 704.

Predictor engine 701 is connected to interface controller 708, vector SAD engine 702 and subpel (sub pixel) engine 703.

Subpel engine 703 is connected vector SAD engine 702, predictor engine 701, interpolation reference buffer 704, processing unit 711 and interface controller 708.

Vector SAD engine is connected to predictor engine 701, subpel engine 703, processing unit 711 and interface controller 708.

Skip engine 706 is connected to interface controller 708, skip input buffer 707, skip MB buffer 705 and programming interface 709.

Buffers 704, 705 and 707 allow the implementation of global search, local integer search and local sub-pixel search operations in parallel. With this architecture, searches in multiple reference frames can also be performed in pipelined fashion, as shown in Table 2. This table shows L0 and L1 (forward and backward) direction search in B frame processing. After completing predictor search (global search) in L0 direction, local integer search (also called vector search) in L0 direction and global search in L1 direction happen in parallel. Similarly, local integer search in L1 direction happens in parallel with local sub pixel search in L0 direction.

A separate sub pixel interpolation and cost calculation engine 703 is used to calculate skip and direct mode costs as defined in H.264 standard.

TABLE 2 Predictor Vector Sub Pixel Search Search Search Skip SAD MBnL0 MBnL1 MBnL0 MBnL0 MBnL1 MBnL0 MBnL1 MBnL1 MBn + 1 L0 MBn + 1 L1 MBn + 1 L0 MBn + 1L0 MBn + 1 L1 MBn + 1 L0 MBn + 1L1 MBn + 1 L1

All the hardware engines operate in streaming fashion, i.e., the reference data fetch, comparison against the current macro-block to calculate the cost, and comparison with the previous best candidates happen in streaming fashion without requiring additional cycles and intermediate storage.

In addition, in the 2 macro block (MB) pipeline mode shown in Table 3, the architecture also supports global search for MBn+1 in parallel with local search for MBn. This significantly enhances the hardware efficiency.

During global search for MBn+1 with a 2 MB pipeline, motion vector of MBn is not yet available. This motion vector is used for calculating motion vector prediction (mvp) which is used for motion vector cost calculation during search. Hence, during global search phase, an approximated mvp is used (MBn's mvp is used by MBn+1). However, during local search phase, a more accurate mvp is used.

TABLE 3 Predictor Vector Sub Pixel Search Search Search Skip Sad MBnL0 MBn + 1 L0 MBnL1 MBnL0 MBnL0 MBn + 1 L1 MBnL1 MBnL1 MBn + 1L0 MBn + 2 L0 MBn + 1L1 MBn + 1L0 MBn + 1L0 MBn + 2 L1 MBn + 1L1 MBn + 1L1

For optimizing memory bandwidth, the architecture uses local buffers 704, 705 and 707 to store the reference data required for portions of the search. The local buffers also free up memory ports and thus enable more parallel operations.

During local search, as shown in FIG. 8, the same reference pixels are repeatedly accessed from the memory 801 by the search engine 804. We take advantage of this fact and store all the data that may be required to perform the local search in custom designed local buffers 803 inside the search engine. Since the local search pattern is be known ahead of time, a bounded region of reference pixels required for local search around the starting point of local search (referred as LS_start_mv) can be identified.

After performing global search, before starting local search this bounded region is fetched from the memory and stored inside a buffer local to the motion estimation engine. Entire local search is then carried out by accessing reference data from the local buffer instead of fetching from the main memory. This significantly reduces the amount of data required to be fetched from the memory, thereby saving power and freeing up memory bandwidth.

More than one instance of local buffers may be included in the design to store more than one reference region required for local search. These reference regions may belong to either the same or to different reference frames.

In addition to storing the integer pixel reference data, the local buffer can also be used to store sub-pixel reference data. For example, H.264 standard specifies a 6-tap filter for half pixel calculation and 2-tap filter for quarter pixel calculation. For quarter pixel calculation, half position pixels are required. During the search operation, typically quarter pixel search is performed after half pixel search. The half pixels calculated during half pixel search can be stored in the local buffer so that during quarter pixel search, half pixel positions need not be recalculated. 

What is claimed is:
 1. A method for processing frames of video data using inter frame prediction, the method comprising: for each macroblock (MB) of a current frame of the video data: during a first processing cycle, performing a first global search of MBs in one or more reference frames in a first direction to identify one or more first candidate MBs; during a second processing cycle occurring after the first processing cycle, performing, in parallel: a second global search of the MBs in the one or more reference frames in a second direction to identify one or more second candidate MBs; a first local search based on the one or more first candidate MBs; and a first determination of a sum of absolute difference (SAD) value between the MB and each of the one or more first candidate MBs; during a third processing cycle occurring after the second processing cycle, performing, in parallel: a second local search based on the one or more second candidate MBs; and a second determination of a sum of absolute difference (SAD) value between the MB and each of the one or more second candidate MBs; determining a motion vector that indicates a selected MB that is selected from the one or more first candidate MBs and the one or more second candidate MBs, wherein the selected MB is an MB that best matches the MB of the current frame of the video data and is determined based at least on the results of the first and second global searches, the first and second local searches, and the first and second SAD values; and using the motion vector to encode the MB of the current frame using inter frame prediction.
 2. The method of claim 1, wherein the first, second, and third processing cycles are consecutive processing cycles.
 3. The method of claim 1, wherein performing the first local search based on the one or more first candidate MBs includes searching a region around each of the one or more candidate first MBs, and wherein performing the second local search based on the one or more second candidate MBs includes searching a region around each of the one or more candidate second MBs.
 4. The method of claim 1, further comprising: performing a first sub-pixel search based on the one or more first candidate MBs; and performing a second sub-pixel search based on the one or more second candidate MBs.
 5. The method of claim 4, wherein the selected MB indicated by the motion vector is further determined based on the results of the first sub-pixel search and the second sub-pixel search.
 6. The method of claim 4, wherein performing the first sub-pixel search based on the one or more first candidate MBs is performed during the third processing cycle in parallel with performing the second local search based on the one or more second candidate MBs and performing the second determination of the sum of absolute difference (SAD) value between the MB and each of the one or more second candidate MBs.
 7. The method of claim 6, wherein performing the second sub-pixel search based on the one or more second candidate MBs is performed during a fourth processing cycle occurring after the third processing cycle.
 8. The method of claim 7, wherein the first, second, third, and fourth processing cycles are consecutive processing cycles.
 9. The method of claim 4, wherein the first and second sub-pixel searches are each based on a half-pixel search.
 10. The method of claim 4, wherein the first and second sub-pixel searches are each based on a quarter-pixel search.
 11. The method of claim 1, wherein the one or more first candidate blocks are from a single reference frame.
 12. The method of claim 1, wherein the one or more first candidate blocks are from two or more reference frames.
 13. The method of claim 1, wherein the first direction is a forward direction and the second direction is a backward direction. 